Hi James et al. the densifier does exactly what it says in the name it makes the concrete in the first 5mm or denser by causing a chemical reaction between the silica of the densifier and the lime of the cement. It also seals up the tiny pores in the concrete making it harder and therefore easier to polish. It was a while back I did this and I had to look to the USA for the information and products apart from one company I found later in Earls Court. A great resource is this lot: http://www.concretenetwork.com/densifiers/
It looks like things have moved on a bit now as they have nano particle densifier as apposed to the ordinary lithium silicate based product I used.